1. Field
The subject invention is in the field of bladed propellers, particularly those for aircraft. More specifically it is in the field of propellers which incorporate a generally cylindrical shroud and still more specifically, propellers incorporating such a shroud attached to the propeller blades and rotating with the propeller hub and blades.
2. Prior Art
The patents listed below constitute a sampling of the prior art in this field.
______________________________________ U.S. Pat. Nos.: 855,131 2,091,677 987,624 2,411,271 1,518,501 2,724,544 1,635,966 3,549,272 1,739,866 4,767,270 2,088,802 French Patents: 669,167 951,186 Great Britain: 991,744 1,324,356 Japan: 121,192 (1985) 43,391 (1987) Italy: 585,534 Russia: 1,141,044 ______________________________________
As is well known in the art, all propeller designs involve design compromises based on many factors including cost, weight, size, noise produced and efficiency, all related to the engine (prime mover) power converted to thrust by the propeller. In current state of the art aircraft using propellers the amount of power and therefore the overall performance of an airplane is often limited not by the prime mover power available but by the amount of power which can be converted to thrust within the limitations of propeller size, propeller efficiency and noise produced. All three factors are strongly related to the strengths of the vortices which are generated at the tips of propeller blades. Generally speaking, when the power level is high relative to propeller diameter (i.e. size is held to a minimum for the power level) the vortices are strong and efficiency is sacrificed and noise levels are high. It has long been recognized, as evidenced by the listed prior art, that the design compromise among power level, propeller size, efficiency and noise produced can be improved by enclosing the propeller in a shroud in an attempt to minimize the losses and noise levels associated with tip vortices. Use of prior art shrouding technique with propellers has not proven sufficiently successful to warrant significant commercial success and related widespread use. This lack of success is attributable to (1) the fact that while prior art shrouds tend to diffuse and/or disperse tip vortices at the blade tips, the vortices reform at virtually full strength some distance downstream from the propeller plane, limiting the benefits of the shroud, and (2) the costs of incorporating shrouds more than offset these limited benefits, the costs including manufacturing and use costs and the functional penalties such as weight, aerodynamic drag, etc.
The turbofan engine, which uses a multi-bladed fan in combination with a fixed shroud, is a somewhat related application. However, it has significant characteristic disadvantages and is not adaptable to and will not realize the benefits of using the rotating ring shroud with varying camber and twist according to the subject invention.
It will be understood by those skilled in the art that the function of a shroud in limiting or preventing blade tip vortices and the associated losses in efficiency is dependent on a variety of variable factors, the result being that with fixed geometry blades and rotating ring-shroud the performance will be optimum at a functional design point and tending to degrade as the function moves away from the design point. This tendency to degrade can be significantly lessened by the use of variable geometry of the blades and shroud, blade geometry varying in pitch angle and shroud geometry varying in camber and twist. Also, for maximum successful utilization, the shroud must be adaptable to use on a variety of types of propellers, including counter-rotating sets. Also, it will be advantageous if the shroud is adaptable for use of more than one shroud on a propeller.
Accordingly, it is a prime objective of the subject invention to provide a shrouded propeller which prevents development of blade tip vortices at a performance design point. A second objective is to provide a ring-shrouded propeller which is adaptable to incorporation of variable geometry such that development of blade tip vortices is prevented and/or inhibited over a range of performance around the design point. A further objective is that the ring-shrouded propeller and the shroud be adaptable to use of more than one shroud on a propeller. Also it is an objective that the ring-shrouded propeller be adaptable for use in dual counter-rotating propeller installations and propeller/stator combinations.